Copied from upstream nettle git repository.
Removed changes to ChangeLog, to allow this patch to apply to nettle-3.5.

From 7616541e6eff73353bf682c62e3a68e4fe696707 Mon Sep 17 00:00:00 2001
From: Niels Möller <nisse@lysator.liu.se>
Date: Thu, 6 May 2021 21:29:56 +0200
Subject: [PATCH] Add check that message length to _pkcs1_sec_decrypt is valid.

* pkcs1-sec-decrypt.c (_pkcs1_sec_decrypt): Check that message
length is valid, for given key size.
* testsuite/rsa-sec-decrypt-test.c (test_main): Add test cases for
calls to rsa_sec_decrypt specifying a too large message length.
---
 ChangeLog                        |  7 +++++++
 pkcs1-sec-decrypt.c              |  4 +++-
 testsuite/rsa-sec-decrypt-test.c | 17 ++++++++++++++++-
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/pkcs1-sec-decrypt.c b/pkcs1-sec-decrypt.c
index 4f13080e..16833691 100644
--- a/pkcs1-sec-decrypt.c
+++ b/pkcs1-sec-decrypt.c
@@ -63,7 +63,9 @@ _pkcs1_sec_decrypt (size_t length, uint8_t *message,
   volatile int ok;
   size_t i, t;
 
-  assert (padded_message_length >= length);
+  /* Message independent branch */
+  if (length + 11 > padded_message_length)
+    return 0;
 
   t = padded_message_length - length - 1;
 
diff --git a/testsuite/rsa-sec-decrypt-test.c b/testsuite/rsa-sec-decrypt-test.c
index fb0ed3a1..3419322e 100644
--- a/testsuite/rsa-sec-decrypt-test.c
+++ b/testsuite/rsa-sec-decrypt-test.c
@@ -55,6 +55,7 @@ rsa_decrypt_for_test(const struct rsa_public_key *pub,
 #endif
 
 #define PAYLOAD_SIZE 50
+#define DECRYPTED_SIZE 256
 void
 test_main(void)
 {
@@ -63,7 +64,7 @@ test_main(void)
   struct knuth_lfib_ctx random_ctx;
 
   uint8_t plaintext[PAYLOAD_SIZE];
-  uint8_t decrypted[PAYLOAD_SIZE];
+  uint8_t decrypted[DECRYPTED_SIZE];
   uint8_t verifybad[PAYLOAD_SIZE];
   unsigned n_size = 1024;
   mpz_t gibberish;
@@ -99,6 +100,20 @@ test_main(void)
                                     PAYLOAD_SIZE, decrypted, gibberish) == 1);
       ASSERT (MEMEQ (PAYLOAD_SIZE, plaintext, decrypted));
 
+      ASSERT (pub.size > 10);
+      ASSERT (pub.size <= DECRYPTED_SIZE);
+
+      /* Check that too large message length is rejected, largest
+	 valid size is pub.size - 11. */
+      ASSERT (!rsa_decrypt_for_test (&pub, &key, &random_ctx,
+				     (nettle_random_func *) knuth_lfib_random,
+				     pub.size - 10, decrypted, gibberish));
+
+      /* This case used to result in arithmetic underflow and a crash. */
+      ASSERT (!rsa_decrypt_for_test (&pub, &key, &random_ctx,
+				     (nettle_random_func *) knuth_lfib_random,
+				     pub.size, decrypted, gibberish));
+
       /* bad one */
       memcpy(decrypted, verifybad, PAYLOAD_SIZE);
       nettle_mpz_random_size(garbage, &random_ctx,
-- 
2.31.1

